1. Field of the Invention
This invention relates to an optical recording method of recording information in a recording layer by irradiating a light beam thereto to induce a crystallographic change and an optical recording medium used therein.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting. One typical rewritable optical recording medium is of phase change type wherein laser light is directed to the recording layer to change its crystallographic state whereby a change of reflectivity by such a crystallographic change is detected. Optical recording media of the phase change type are of great interest since they can be overwritten by a single light beam and operated by a drive unit with a relatively simple optical system as compared with magnetooptical recording media.
Most optical recording media of the phase change type used Ge--Te systems which provide a substantial difference in reflectivity between crystalline and amorphous states and have a relatively stable amorphous state. It was recently proposed to use new compounds known as chalcopyrites. Chalcopyrite type compounds were investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite type compounds are composed of Ib-IIIb-VIb.sub.2 or IIb-IVb-Vb.sub.2 as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite type compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in "Physics", Vol. 8, No. 8 (1987), pp. 441 and Electrochemistry, Vol. 56, No. 4 (1988), pp. 228.
Among the chalcopyrite type compounds, AgInTe.sub.2 is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s. See Japanese Patent Application Kokai (JP-A) No. 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992.
In addition to these phase change type optical recording media using chalcopyrite type compounds, JP-A 267192/1992 and 232779/1992 disclose phase change type optical recording media wherein a recording layer crystallizes to create an AgSbTe.sub.2 phase.
In the phase change type optical recording media, it is important that not only a C/N ratio and a modulation factor are excellent immediately after recording, but also they do not deteriorate during shelf storage. However, those phase change type optical recording media wherein recorded marks are amorphous or microcrystalline tend to deteriorate in C/N and modulation factor during high-temperature storage because the recorded marks gradually crystallize. It is thus desired to have more reliable such media.